The Total Wipeout host confirmed at the National Television Awards on Wednesday (January 25) that her three-and-a-half-year romance with the now-retired sportsman had come to an end.
She declined to comment further on the matter, saying only that she was "gutted" about the break-up.
McMullen proposed to Byram after she completed a charity trek through Peru with the likes of Denise van Outen and Fearne Cotton in September 2010.
Byram said the previous April that she was "getting ready" to start a family with McMullen.
"The next step is marriage and kids," she said. "I want to be physically prepared, I'm ready to be a mum."
Byram, who is currently working with British Military Fitness, has today (January 29) been tipped to return to a revamped edition of Channel 4 show The Big Breakfast, apparently slated to air in time for the London 2012 Olympics.